Switch failure monitoring device

ABSTRACT

A switch failure monitoring device comprising two switches which are ganged together so that they operate substantially simultaneously; should either of them function incorrectly then a relay removes potential from the load.

United States Patent 11 1 1111 3,857,067 Zanboor ie 1451 Dec. 24, 1974SWITCH FAILURE MON1TOR1NG DEVICE [56] References Cited [75] Inventor:Mohammed H. Zanhoorie, UNITED STATES PATENTS Rockvue 2,594,520 4/1952Tiedman 192/131 R 2,882,456 4/1959 Koch 317/135 R [73] Asslgnee' I UnwedStates of Amenca as 2,962,633 11/1960 Raymond 317 135 R presented by theSecretary of the Department of Heaith Education and :3 x3 I welfare,Washington DC.

1 1 Filedl EC. 13, 1972 Primary Examiner-J. D. Miller AssistantExaminer-Harry E. Moose, Jr. [21 1 Appl' 314,508 Attorney, Agent, orFirmBrowdy and Neimark [52] Cl. 317/9 A, 2l9/10.55, 317/36 TD,

317/135 R [57] ABSTRACT [51] Int. Cl. H0211 3/00 A switch failuremonitoring device comprising two Field of Search 317/9 D, 135, 141 R,154, switches which are ganged together so that they oper- 317/9 R, 9 A,18 R, 18 A, 101 CB, 136; 340/248 R, 248 E, 256; 192/131 R; 219/695,10.55; 307/141.8, 142, 115, 116, 136, 140;

ate substantially simultaneously; should either of them functionincorrectly then a relay removes potential from the load.

5 Claims, 3 Drawing Figures Patented Dec. 24, 1974 3,857,067

II I2 1 I3 POWI-R MONITORFL) RELAY SWITCH I l I H6 TIME DELAY MONITORINGRELAY SWITCH 1 I I4 l5 FIGURE lb FIGURE 2 SWITCH FIGURE l0 (Prior AFT)SWITCH FAILURE MONITORING DEVICE FIELD OF THE INVENTION BACKGROUND OFTHE INVENTION quired to initiate an operation and hence are out of thedanger area. Since it is inherently impossible for a person to actuatetwo switches simultaneously, it is common practice in control systems ofsuch kind to provide a means for establishing a time interval duringwhich the two switches must be actuated to initiate the operation.

However, it has been found that in practice the desired safety is notachieved with prior art constructions. For example, certain operatorswill cheat the system and maintain one of the switch units in apermanently closed position, such as by stretching a piece of adhesivetape or the like across one of the switch units in a positionmaintaining the operating button thereof depressed, so that in this wayone of the switch units is always maintained closed, and with such anexpedient an operator can have one hand free so that he still can beinjured.

Additional problems can occur in the failure of prior control devices.Thus, recent failure of interlocks in cabinet radiographic units haveresulted in a severe damage to the operators. Failure of the safetyinterlocks on some microwave ovens of the same model have resulted inrecall" of these units including a massive program to locate these ovensand to correct the failure before serious damage occurs to users.

SUMMARY OF THE INVENTION The present invention overcomes many of theweaknesses and shortcomings of similar prior art safety devices byproviding an effective and foolproof monitoring device to insure thatthe operation of two switches is performed properly to thereby protectpersonnel.

It is, accordingly, an object of the present invention to overcome thedefects of the prior art, such as indicated above.

It is another object to protect operating personnel of microwave ovensand cabinet radiographic units.

It is another object of the present invention to provide an improvedsafety interlock system.

Another object of the present invention is the provision of a switchmonitoring device to protect operating personnel.

Still another object of the invention is the provision of a monitoringswitch to monitor another switch, both of which must be operatedsubstantially simultaneously.

Yet another object of the invention is the provision of a monitoringdevice having a time delay means for removing potential from themonitored switch and its load should either switch function improperly.

Another object of the invention is the provision of a switch monitoringdevice which, in case of improper operation, removes potential from theload and maintains the disconnection until the fault is corrected.

The present invention is very important from a public health standpoint.Thousands of microwave ovens, cabinet radiograph units, and many othertypes of equipment require the use of interlock safety switches toprotect the user from accidental exposure to radiation.'

BRIEF DESCRIPTION OF DRAWING Other objects and many of the attendantadvantages of the invention will be readily appreciated as the samebecomes better understood by reference to the following detaileddescription of an embodiment when considered in connection with theaccompanying drawings in which like reference numerals designate likeparts throughout the figures thereof and wherein FIG. lla shows a simpleblock diagram of a part of a system without the present invention.

FIG. llb shows a block diagram of a system like that of FIG. la to whichan embodiment of the invention has been added.

FIG. 2 shows a circuit diagram of an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENT Referring to the drawings, there isshown in FIG. 1a a prior art system including an input terminal 10 forthe application of potential from within the system, through a switch12, to an output terminal 13. The output terminal 113 is connected tosome form of load or energy consuming device within the system, notshown, such as theradiation generating load of a microwave oven or thatof a cabinet radiographic unit.

Looking now at the drawing of FIG. 1b, there is shown the same inputterminal for the application of potential to the device comprising theinvention. Connected to the input terminal 10 is a power relay ll, ofany conventional design, and the output of this relay is seriesconnected to a monitored switch 12, which may be either single or doublepole, to apply potential to an output terminal 13 connected, as in FIG.1a, to a load or energy consuming device.

Located to form a circuit connected to power relay ill, and monitoredswitch 12, is another series circuit comprising a time delay relay 14,of any conventional designs, and a monitoring switch 15, the time delayrelay 14 being connected to the power relay 11 and the monitoring switch115 being connected to monitored switch 12. A common barrier-typeconnection 16 joins monitored switch 12 and monitoring switch 15 toaffect substantially simultaneous operation of the two switches. Thusunless the monitored switch 12 performs in a proper manner when it isoperated, the monitoring switch 115 functions to remove potential fromthe load, as will be described more fully hereinafter.

Looking now at the circuit diagram of FIG. 2, it will be noted that thecomponent parts bear the same identifying numerals as used in FIG. ll.Switches 12 and 15 each have one set of contacts. Each set of contactshas a common terminal C and two output terminals which are crossconnected so that, in the energized condition as shown in FIG. 2,monitored switch 12 normally applies potential to output terminal l3,and monitoring switch 15 normally does not apply potential to time delayrelay 14. Should monitored switch 12 not operate, but monitoring switch15 does operate, then potential is applied to time delay relay l4, andconversely, if switch 12 operates properly but switch 15 does not, thenagain the time delay relay 14 is energized.

The time delay relay 14 is so designed as not to function if it isenergized only briefly, as by transients, but will reliably perform ifthe potential persists past a predetermined time duration. Relay 14(shown in its unenergized condition in FIG. 2) has a coil and time delaymechanism designated by 18 and one set of contacts; one contact terminalof this set of contacts is left unconnected, the other contact terminalis connected to the coil of power relay 11, and the contactor armterminal C is connected to the potential terminal 10, so that when it isenergized the delay relay 14 applies power to the activating coil 19 ofthe power relay 11.

Power relay 11 has two sets of contacts, A and B, with the commonterminal C of each set connected to input terminal 10. In itsunenergized condition, as shown in FIG. 2, one contact terminal of set Aapplies potential from the potential terminal to the common terminal Cof switch 12, and the other contact terminal is left unconnected; onecontact terminal of set B is left unconnected, and the other contactterminal is connected to its relay activating coil 19. As will beunderstood hereinafter, when contacts B are closed, power relay 11 willremain activated as long as potential is applied to input terminal 10.

Under normal conditions, when the two switches 12 and are operatingproperly, current passes through normally closed contacts A of theunenergized power relay 11, through monitored switch 12, through outputterminal 13, and on to the load.

The switch failure monitoring device of the present invention is builtso that the two separate switches, the monitoring and the monitored,must be mechanically actuated at approximately the same time. Thisactuation renders the contactors of each switch as shown in theschematic diagram of FIG. 2, in the same mode,

i.e., both are in normally open or in normally closed position, and thusline voltage will normally not appear across the time delay relay.During the transition period, when both switches are in the process ofbeing actuated, the line voltage will momentarily be applied to the timedelay relay until both switches are properly actuated. The length oftime in which power is applied to the delay relay in this way isnormally not long enough to activate it. However, if either switchfailed to operate properly, so that the contactors are not in the samemode, line voltage will be applied to the time delay relay.

When the specified time required for the time delay relay to operate haselapsed, it will close its contacts and apply the line voltage to thepower relay. The power relay will thus be energized causing its contactsA to open and thereby cut out the power supplied to the load, whilecausing its normally open contacts B to close and thereby sustain theline voltage across its coil so that the power relay is kept energized.The power relay will remain energized as long as the line voltage isapplied to its coil. Therefore, if any of the two switches are notfunctioning normally, the switch failure monitoring device will renderthe system inoperative until the cause of the failure is corrected.Also, if an operator attempts to cheat" the system by tapeing eitherswitch 12 or 15 closed, then the power relay will be energizedpreventing application of power to the output terminal 13.

It should be noted that when the line voltage is disconnected from thecoil of the power relay, the monitoring device will automatically resetitself and will function as described above when the line voltage isreconnected. The elements of the system are all available and a suitabletime delay relay having a desired time lag may be selected from thoseavailable.

From the above description of the structure and operation of the presentinvention it is obvious that the device provides many improvements overthe weaknesses and shortcomings of similar prior art systems. Thus, theinvention offers reliable, speedy, and adequate protection to operatingpersonnel so that they will not be endangered by failure of removal ofpotential from equipment which was thought to be unenergized.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that the invention may be practiced otherwise than asspecifically described.

What is claimed is:

1. An electrical circuit for deenergizing a load device in case ofimproper functioning in microwave ovens and cabinet radiographic unitscomprising:

an operating circuit;

a monitoring circuit connected to the operating circuit;

first means in the operating circuit for removing potential therefrom;

first switch means in the operating circuit for applying potential to aload device, said first means being a power relay connected in serieswith said first switch;

second switch means in the monitoring circuit; and

time delay means in the monitoring circuit connected between the secondswitch and the power relay and arranged to energize the first meansshould either the first or second switches fail to function or shouldone of said switches be activated prematurely irrespective of whether acontinuity through the load is maintained.

2. The device of claim 1 wherein the first and second switches areactuated by a common barrier for substantially simultaneous operation.

3. The device of claim 2 wherein the time delay means is a relay whichoperates after it has been energized for a predetermined length of time.

4. The device of claim 3 wherein the power relay has a plurality ofcontacts, one set of which removes potential from the operating circuit,and another set maintains potential on the relay.

5. A safety circuit comprising:

a power relay connected to a source of potential;

a monitoring switch connected to the power relay for applying potentialto a load;

a monitor switch connected by a common barrier to the monitored switchso that the switches operate substantially simultaneously; and

a time delay relay connected between the monitoring switch and the powerrelay whereby should either switch fail to function normally or beprematurely activated the time delay relay will energize the power relayto remove potential from the load irrespective of whether a continuitythrough the load is maintained.

a =r r

1. An electrical circuit for deenergizing a load device in case ofimproper functioning in microwave ovens and cabinet radiographic unitscomprising: an operating circuit; a monitoring circuit connected to theoperating circuit; first means in the operating circuit for removingpotential therefrom; first switch means in the operating circuit forapplying potential to a load device, said first means being a powerrelay connected in series with said first switch; second switch means inthe monitoring circuit; and time delay means in the monitoring circuitconnected between the second switch and the power relay and arranged toenergize the first means should either the first or second switches failto function or should one of said switches be activated prematurelyirrespective of whether a continuity through the load is maintained. 2.The device of claim 1 wherein the first and second switches are actuatedby a common barrier for substantially simultaneous operation.
 3. Thedevice of claim 2 wherein the time delay means is a relay which operatesafter it has been energized for a predetermined length of time.
 4. Thedevice of claim 3 wherein the power relay has a plurality of contacts,one set of which removes potential from the operating circuit, andanother set maintains potential on the relay.
 5. A safety circuitcomprising: a power relay connected to a source of potential; amonitoring switch connected to the power relay for applying potential toa load; a monitor switch connected by a common barrier to the monitoredswitch so that the switches operate substantially simultaneously; and atime delay relay connected between the monitoring switch and the powerrelay whereby should either switch fail to function normally or beprematurely activated the time delay relay will energize the power relayto remove potential from the load irrespective of whether a continuitythrough the load is maintained.